The present invention relates to ceramic members, and more particularly to the finish machining of high density ceramic members such as may be used for turbine components. The invention herein described was made in the course of or under a contract, or subcontract thereunder, with the Department of Defense.
High density ceramic materials such as hot pressed silicon nitride and silicon carbide have high strength and the ability to withstand high temperatures. These materials, therefore, are very desirable for components of gas turbines such as vanes, blades and supporting structures, since they permit operation at temperatures as high as 2500.degree. F with resulting improved efficiency and performance of the turbine. Such parts are usually produced by hot pressing the material to the desired configuration and then machining to the exact final dimensions, usually by diamond grinding because of the hardness of the material. Diamond grinding, using a grinding wheel of 220 grit size, for example, is frequently specified for this finishing operation, but the surface finish to be obtained is usually not specified. The finish resulting from this usual practice, therefore, may have a surface roughness varying over a considerable range, which may be of the order of 20 to 50 RMS, depending on such factors as the type of coolant used, the grinding speed and the contact pressure. This conventional practice results in a surface having numerous surface defects such as cracks and scratches in the direction of grinding. The edges and corners are usually sharp, or essentially knife-edges, causing notches, chip-outs and other defects.
These high strength, high density ceramic materials are relatively brittle and are extremely sensitive to external and internal flaws of both microscopic and macroscopic size. In the use of such ceramic members, the maximum stress applied to the member very often occurs at or slightly beneath the surface, and flaws such as those mentioned above are likely to result in a catastrophic failure. Residual stresses are also often established in the vicinity of surface scratches because of temperature non-uniformities resulting from the grinding process. Such residual stresses add directly to the applied stresses and this is a serious problem in ceramic materials because these residual stresses cannot be relieved by creep or plastic flow as in materials such as metals.
Turbine components such as vanes, blades and shrouds are subjected to both steady-state and transient stress conditions in use, and these stresses are frequently tensile stresses and are very often at their maximum at the outer surface of the component. Ceramic materials usually have their greatest strength in compression and are relatively weaker in tension so that these surface tensile stresses are very critical. Surface defects in the materials such as those discussed above act as stress concentrations which add directly to the applied stresses and these stress concentrations can reduce the strength of the material by 40% to 50%. Obviously, such a reduction in strength cannot be tolerated in the design of these components. The same problem exists in the preparation of test specimens and if surface defects of this type exist on specimens for physical testing, they prevent accurate determination of the true mechanical properties of the material.
It has been believed that the detrimental effects of this surface damage and the residual tensile stresses will be reduced after many hours of operation at high temperature as in a gas turbine. However, adequate performance of the ceramic components must be obtained through severe modes of operation, such as installation, start-up and shut-down of the machine, prior to achieving the many hours of high temperature operation which might cause an improvement in the detrimental effects of the surface faults. Such a long delay in the attainment of the required strength, even if it occurs, cannot be considered acceptable performance.
It has also been thought that after a sufficient period of operation, the effects of surface finish of the ceramic parts will not be important because of the oxide film formed on the surface of the material. Such a film, however, does not really help the situation because mechanical differences, such as differences in thermal expansion, strength and elasticity between the oxide film and the ceramic material, are such that the film is very likely to craze and flake off so that it would not serve any useful purpose. Such crazing itself can induce cracks which readily propagate to scratches in the surface of the material and can precipitate a failure in this way. There are also, of course, highly stressed surface areas on which a protective coating of this kind cannot form or be retained, such as contact surfaces where any coating would be immediately ground off. The presence of surface defects of the kinds described above, therefore, which result in a reduction in the effective strength of the ceramic material, is not acceptable and their effects are not effectively obviated even after the ceramic part has been in service. The ceramic components for gas turbine use finished in the conventional manner are, therefore, not entirely satisfactory for exhibiting the ability to withstand the tensile stresses they are subjected to.